Recent requirements for reduction in a weight and a size in electronics have led to development in direct mounting and high-density mounting of semiconductor components on a printed wiring board. Resultantly, higher quality has been required for a laminated board used in a printed wiring board on which semiconductor components are mounted.
A laminated board with both sides being metalized has been conventionally prepared by attaching metal foils such as copper foils on both sides of an insulator layer formed by coating a base material layer such as a glass cloth with a thermosetting resin such as an epoxy resin or a phenol resin or on both sides of a laminate formed by laminating the above plurality of insulator layers, as an integral part. Such a laminated board with both sides being metalized is generally manufactured by multiple batch press where a long base material layer is sequentially impregnated with resins and sequentially dried to form a long prepreg; the long prepreg is cut into pieces with a given size to prepare standard-length prepregs; a given number of the standard-length prepregs are laminated; metal foils having the same size as that of the standard-length prepreg are attached on both sides of the laminate of the given number of the standard-length prepregs; and then pressed under heating a plurality of copper foils, prepregs, printed circuit boards for an inner layer, mirror plates and so on between hot platens to cure the resins.
However, in such a multiple batch press, depending on a position in a hot platen of the laminated board, heat history in each laminated board varies during laminate molding, resulting in difference in qualities such as moldability, warpage and dimensional change, which makes it difficult to supply a product with less quality variation.
Furthermore, it is necessary to press a laminated board under a high pressure of 20 to 100 kg/cm2 for removing air remaining between base material layers, leading to the problem of insufficient thickness accuracy due to resin flow.
It is believed that the limit of a thickness of an insulator layer in a laminated board with both sides being metalized is 60 μm in a conventional batch press, and such a level cannot meet the needs for a thinner laminated board with both sides being metalized which is required in the industry with tendency to weight- and size-reduction; specifically, if the thickness is less than 60 μm, the base material tends to be directly in contact with the metal foils on both surfaces and reliability in insulation is reduced due to air remaining between the base material layers.
Furthermore, the multiple batch press using a prepreg requires many steps as described above, leading to a complex process and reduced productivity.
In a conventional method for preparing a prepreg, for example a method where a fiber cloth base material is immersed in a resin varnish for impregnation and dried using a common coater, streaky asperity tends to generate in a coating direction, making it difficult to ensure thickness precision.
In a method using a roll laminator, thickness precision in an insulating layer formed can be controlled, a desired insulating layer can be easily formed, and the method can be continuously conducted, resulting in an efficient method in terms of productivity. Thus, for a method using a roll laminator, the use of a prepreg exhibiting good thickness precision and impregnation property would be effective.
However, in a method using a roll laminator, a plurality of prepreg are continuously laminated, so that setting the manufacturing conditions is difficult and void generation in an insulating resin layer becomes significant. Again, when a roll laminator is used, streaky asperity tends to generate in a coating direction, making it difficult to ensure thickness precision.
To solve these problems, there has been disclosed, as a method for preparing a prepreg exhibiting good thickness precision, a method for laminating insulating resins with a carrier on both sides of a fiber cloth base material (see, for example, Patent Reference No. 1). Patent Reference No. 1 has described that a prepreg exhibiting excellent thickness precision can be prepared by a method for laminating insulating resins with a carrier on both sides of a fiber cloth base material.
Patent Reference No. 1: Japanese published unexamined application No. 2004-123870.
Patent Reference No. 2: Japanese published unexamined application No. 2001-138437.
Patent Reference No. 3: Japanese published unexamined application No. 2005-262591.